Autofocus mechanisms for cameras are well known. Such autofocus mechanisms typically move one or more lenses of a camera so as to achieve desired sharpness or focus of an image. Autofocus mechanisms can be either active or passive. Active autofocus mechanisms determine a distance to the subject and then move one or more lenses to a position that has been determined to provide proper focus for that distance. Passive autofocus mechanisms determine the sharpness of images as the focusing lens(es) are moved, so as to determine the particular position where the best focus is obtained.
The distance to the subject can be determined by measuring the time that it takes for a signal, such as infrared light or ultra high frequency sound, to travel to the subject then and bounce back to the camera. Triangulation or the amount of light reflected by a scene can also be used to determine distance.
Sharpness of the image can be determined by computer analysis. The lens or lenses are moved while the computer measures the sharpness of each image produced during the movement. The sharpest image can be considered to be the best focused imaged.
To determine sharpness, the computer can look at the difference in intensity between adjacent pixels of an imaging sensor. The frame that provides maximum intensity difference, at least for some portion of the image, is considered to be the best focus.
With either active or passive focusing, a selected portion of the image can be used to determine focus. For example, when a person is being photographed, the person is generally centered in the image. Thus, using the central portion of the image tends to result in the most desirable focus.
Although such autofocus mechanisms have proven generally suitable for their intended purpose, they do possess inherent deficiencies that tend to detract from their overall desirability. For example, contemporary digital autofocus cameras display each frame or image as the autofocus mechanism searches for the best focus. Thus, contemporary digital autofocus cameras typically display a number of unfocused images before displaying the final focused image. During the focusing process, the focus is constantly changing and an image can go in and out of focus several times as the autofocus mechanism attempts to determine the best focus.
Displaying such a series of images having constantly changing focus is undesirable. More particularly, displaying a scene while the focus is changing can be annoying and distracting. The distraction may even cause a user to miss the desired timing of a shot, particularly for a moving subject.
In view of the foregoing, it is desirable to provide a digital autofocus camera that does not display the unfocused images associated with the process used by the autofocus mechanism to determine the best focus while still maintaining a display of the video stream.